1. Field of the Invention
The present invention relates to a variable tuning circuit using a variable capacitance diode and a television tuner provided with the variable tuning circuit.
2. Description of the Related Art
Television tuners are provided with variable tuning circuits using variable capacitance diodes in order to select a received frequency or remove interference waves (for example, see JP-A-10-242805). FIG. 3 is a functional block diagram illustrating the structure of a television tuner disclosed in JP-A-10-242805. In FIG. 3, television radio wave signals received by an antenna 101 are tuned by a tuning circuit 102, and the tuned signals are amplified by a high-frequency amplifying circuit 103. Then, a double tuned circuit 104 selects a received frequency, and the selected frequency is input to a frequency mixing circuit 105. The frequency mixing circuit 105 mixes the received frequency with a local oscillation signal that is output from a local oscillating circuit 106, and the mixed frequency is converted into an intermediate frequency, which is the difference between the received frequency of the double tuned circuit 104 and the local oscillation signal that is output from the local oscillating circuit 106. Then, the converted frequency is output from the frequency mixing circuit 105. The intermediate frequency is amplified by an intermediate frequency amplifying circuit 107 and then output from the television tuner.
In the television tuner, a variable tuning circuit is provided in each of the tuning circuit 102, the double tuned circuit 104, and the local oscillating circuit 106. FIG. 4 is a diagram illustrating the structure of a general variable tuning circuit. As shown in FIG. 4, a varactor diode D1 is connected in parallel to an inductor L1, and a tuning voltage Vc is applied to a cathode of the varactor diode D1. In addition, in FIG. 4, a direct current cut-off capacitor C having a large capacitance is connected to the cathode of the varactor diode D1. In this structure, a resonant frequency is changed by varying the capacitance of the varactor diode D1 according to the level of the tuning voltage Vc applied to the varactor diode D1, thereby selecting a desired frequency.
FIG. 5 is a characteristic diagram illustrating a capacitance curve indicating a variation in capacitance according to a voltage applied to the varactor diode. On the basis of the capacitance curve shown in FIG. 5, a tuning frequency is determined by the inductor L1 that is connected in parallel to the capacitor C connected in series to the varactor diode D1. It is necessary to reduce both the value of the inductor L1 and the capacitance value in order to ensure tuning characteristics in a high frequency band. In order to ensure the tuning characteristics in a low frequency band, the capacitance of the varactor diode D1 varies while fixing the value of the inductor L1 and the capacitance value, which are reduced in order to ensure the tuning characteristics in the high frequency band.
However, since various capacitances, such as the stray capacitance of a mounting substrate and the internal capacitances of various types of semiconductor devices, exist in parallel to the varactor diode D1 in the television tuner, the variable range of the high frequency band tends to be narrowed in the variable tuning circuit.
Meanwhile, in the low frequency band of the variable tuning circuit, the value of L of the inductor L1 needs to be set to a small value, which results in a reduction in the value of Q of a circuit. For example, in the tuning circuit 102, impedance is lowered, and thus loss and noise increase, resulting a low gain. In addition, in the local oscillating circuit 106, the value of Q of the circuit decreases, which results in unstable oscillation.